In household and commercial usage, most hot water heaters in North America, as well as numerous other countries, are of a storage type. Such storage type water heaters typically include a cylindrical vessel/container (i.e., tank) in which water is kept continuously hot and ready for use. Typical sizes for household use range from about 20 to 120 U.S. gallons. Water heaters for commercial applications are often much larger. Heating the water in the tank may is typically affected by way of electrical heating elements or fossil fuel burners (e.g., natural gas, propane, fuel oil, bio mass, etc.) burners. Often, the source of the energy for heating is a local utility.
In most hot water heaters, water is introduced into the storage tank via a water supply inlet pipe. Water typically enters residences in the US at about 50° F. though this varies with latitude of the residence and the season. An electrical element within the tank or a burner disposed beneath the tank is selectively operated to heat the water within the tank to a preset temperature. For instance, hot water temperatures of 105-120° F. are typically preferred for residential applications. Operation of the element or burner is controlled by a thermostat that monitors/measures the temperature within the tank. When the water within the tank is below a desired temperature, the electrical element is energized or the burner is ignited to heat the tank and the water therein.
Energy efficiencies of water heaters in residential use can vary greatly based on manufacturer, model and/or age. Gas fired water heaters typically have efficiencies of between about 55% and 95%. Electric water heaters have high efficiencies (e.g., above 95%) once the electricity enters the home but are more costly to operate due to electricity generation and transmission costs. In either case, a typical household expends roughly 30% of its total energy use heating water.